As our society becomes more mobile and information dependent the demand for untethered data communications continues to grow. A proliferation of radio data communications systems have been, are being, or will be deployed in an attempt to satisfy this demand. To no ones surprise, these systems, while often occupying or providing coverage to overlapping geographic areas and user groups, do not always lend themselves to providing, alternatively, relatively seamless data message delivery service to a particular subscriber's unit or terminal.
The systems, as deployed and planned, have sought to optimize different criteria depending on the system designers perception of what user groups needs are intended to be addressed by a particular radio data communications system. Such criteria have included various combinations of data message delivery capacity and delivery reliability, conservation of radio frequency spectrum, the economics of system deployment and expansion, and the extent of the desired geographic coverage. The process of optimization often relies on or takes advantage of expected or measured characteristics and content of data messages that are representative of probable system traffic. As a result of the above considerations at least two distinct types of systems, often referred to as a single frequency and multiple frequency reuse systems respectively, have evolved to provide data message delivery to user groups throughout a geographic area. This evolution is such that the present infrastructure from a collective or macro perspective for any one populous geographic region is now often referred to as a general frequency reuse system.
While distinct, both systems include some similar elements, functions, or characteristics. For example, both systems (networks) likely are centrally managed under the control of a network controller and include a plurality of fixed (base) stations arranged and managed to provide data message delivery to subscriber units or stations (portable or mobile terminals) throughout a geographic area. The network controller includes, among others, a data message routing function for selecting the appropriate path or point of origination, such as a base station, to attempt a data message delivery to a particular subscriber station. This path selection will depend in part on an estimate of the geographic location of the particular subscriber station or other system activity and may include when to attempt a data message delivery, which base station to utilize, and therefore, or additionally, which radio channel (a radio channel may represent two radio frequencies, one for receive and one for transmit).
One of these systems, referred to as a multi-frequency reuse (MFR) system, is characterized by typically comparatively small coverage areas with adjacent areas employing different radio channels, thus frequencies, and spatially distant areas reusing the same radio channels. The areas in total provide coverage throughout the intended MFR geographic area. Ordinarily the fixed stations, at least one per area, in this system are continuously transmitting and receiving and subscriber stations, such as portable or mobile stations or units are capable of operating on any legitimate and authorized network channel. The portable stations, by scanning the network channels, etc., can determine or aid in determining their location within the intended MFR geographic area on a more or less real time basis by observing the better quality channels based on signal strength, error rates, etc. The MFR network, although using several radio channels and thus frequencies, can provide significant data message delivery capacity since all areas may be simultaneously and independently active. Said another way, any path within the MFR system will, at least in principle, have a unique radio channel, i.e. frequency or frequency pair.
The second system, referred to as a single frequency reuse (SFR) system, is characterized by a multiplicity of coverage areas where all areas and potential paths are served by the same radio channel. As above, the areas in total provide coverage throughout the intended SFR geographic area. The fixed stations, usually one per area, in the SFR system are not ordinarily all simultaneously and independently active. To demonstrate, since all areas and paths operate on the same radio channel any two or more areas, when simultaneously active (respective fixed stations transmitting), will have an interference region. This region's geographic size and boundary will depend in part on the spatial separation, radio power levels, etc., of the respective base stations. Within this interference region a given portable station likely cannot resolve (successfully receive) a data message from either of the stations unless some further coordination of the base stations is undertaken.
In essence the effective coverage area depends at least in part on activity within other areas of the SFR system. Portable stations used in the SFR system need only operate on the assigned channel for the network and will not be able to directly aid in determining their location within the intended SFR geographic area unless and until an appropriate fixed station is enabled and uniquely identified. The SFR network tends to be viewed as a spectrally efficient and cost effective approach to providing coverage to a comparatively large geographic area. This follows from the limited number of frequencies employed and comparative simplicity of the portable stations, etc. Somewhat offsetting the above noted attributes, resulting from the single channel, interference regions, etc., a SFR system will typically have relatively limited data message delivery capacity and often more complicated data message routing functions.
The characteristics of either of these systems much less the characteristics of a combined system together with the growing popularity of data communications and hence number of subscriber units or stations or terminals mandates that the practitioner in the field pay careful attention to the particulars of a subscriber unit roaming from one geographic area to a second geographic area within a given system or roaming from one system to a second system.
Generally this topic may be called mobility management. Some straight forward approaches to mobility management have been discussed including, for example, at a subscriber unit simply keying up and transmitting some inquiry type message and waiting to determine whether any network responds each and every time your unit is powered up or each time a data channel you are operating on becomes unsatisfactory. Unfortunately this is unsatisfactory because it wastes valuable system capacity without delivering any data messages and further consumes a significant amount of battery capacity if the subscriber unit is battery powered, such as in the case of a portable unit. Additionally, due to the large number of potential channels or frequencies as well as data protocols, the time delay associated with this technique at its most basic renders the approach simply not practical.
Concerns such as these and practical considerations such as the fact that the subscriber unit rather than the network is likely to best know what data communications attributes and geographic particulars best serve the subscriber's needs as well as the economics of manufacturing a `standard` subscriber unit has resulted in various other mobility management techniques that may be employed by subscriber units. Such techniques include a preloaded list of channels to scan or manually select when the presently used channel no longer provides adequate connectivity and must therefore be exited. Another includes a continual scan of possible frequencies on which to register and operate so that if the present registered channel degrades an undo amount the subscriber unit may exit such channel and may begin registration attempts on the other possible channels or frequencies. All such prior art techniques continue to be unsatisfactory for a generalized subscriber unit, either wasting system capacity, resulting in excess data message delivery latency, or consuming excess power, that is expecting to have data service on any of the plethora of various data communications networks. Clearly an urgent need continues to exist for improved techniques that address mobility management and more particularly methods of making channel acquisition decisions.